Hercules (dwarf galaxy)
Encyclopedia
Hercules, or Her, is a dwarf spheroidal galaxy
situated in the Hercules constellation and discovered in 2006 in data obtained by the Sloan Digital Sky Survey
. The galaxy is located at a distance of about 140 kpc from the Sun and moves away from the Sun with a velocity of about 45 km/s. It is classified as a dwarf spheroidal galaxy
(dSph). Her has a noticeably elongated (ratio of axes ~ 3:1) shape with a half-light radius of about 350 pc. This elongation may be caused by tidal forces acting from the Milky Way galaxy, meaning that Her is being tidally disrupted now. Her also shows some gradient of velocities across the galaxy's body and is embedded into a faint stellar stream, which also points towards its ongoing tidal disruption.
Her is one of the smallest and faintest satellites of the Milky Way
—its integrated luminosity is about 30,000 times that of the Sun (absolute visible magnitude of about −6.6), which is comparable to the luminosity of a typical globular cluster
. However, its total mass is about 7 million solar mass
es, which means the galaxy's mass to light ratio
is around 330. A high mass to light ratio implies that Her is dominated by dark matter
.
The stellar population of Her consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at , which means that they contain 400 times less heavy elements than the Sun. The stars of Her were probably among the first stars to form in the Universe
. Currently there is no star formation in Her. Measurements have so far failed to detect neutral hydrogen in it—the upper limit is 466 solar masses.
Dwarf spheroidal galaxy
Dwarf spheroidal galaxy is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31...
situated in the Hercules constellation and discovered in 2006 in data obtained by the Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...
. The galaxy is located at a distance of about 140 kpc from the Sun and moves away from the Sun with a velocity of about 45 km/s. It is classified as a dwarf spheroidal galaxy
Dwarf spheroidal galaxy
Dwarf spheroidal galaxy is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31...
(dSph). Her has a noticeably elongated (ratio of axes ~ 3:1) shape with a half-light radius of about 350 pc. This elongation may be caused by tidal forces acting from the Milky Way galaxy, meaning that Her is being tidally disrupted now. Her also shows some gradient of velocities across the galaxy's body and is embedded into a faint stellar stream, which also points towards its ongoing tidal disruption.
Her is one of the smallest and faintest satellites of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
—its integrated luminosity is about 30,000 times that of the Sun (absolute visible magnitude of about −6.6), which is comparable to the luminosity of a typical globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
. However, its total mass is about 7 million solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es, which means the galaxy's mass to light ratio
Mass to light ratio
In astrophysics and physical cosmology the mass to light ratio, normally designated with the symbol \Upsilon is the quotient between the total mass of a spatial volume and its luminosity...
is around 330. A high mass to light ratio implies that Her is dominated by dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
.
The stellar population of Her consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at , which means that they contain 400 times less heavy elements than the Sun. The stars of Her were probably among the first stars to form in the Universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
. Currently there is no star formation in Her. Measurements have so far failed to detect neutral hydrogen in it—the upper limit is 466 solar masses.